1. Field of the Invention
This invention relates to methods and apparatus used in the design and manufacture of surfboards, sailboards or similar aquatic boards, referred to generically herein as xe2x80x9cboardxe2x80x9d or xe2x80x9cboards.xe2x80x9d
2. Description of the Related Art
Surfboards and sailboards are of similar shape, however the sailboard is generally manufactured in a mold, while the surfboard is fabricated using a labor-intensive moldless or custom method of construction. The conventional molds used in surfboard and sailboard construction comprise top and bottom halves, with a part line at the point of greatest breadth along the board""s perimeter edge or rail, and utilize the mold""s concave, female surface to define the shape of the board, and impart a smooth surface to the fiber-reinforced plastic skin. Currently available molded production techniques restrict the shape of the board to an exact duplicate; which generally limits molded production to the less demanding design of the sailboard. For molded surfboard production, the very wide variation in size and shape requires the manufacturer to invest in a large and prohibitively expensive inventory of molds, and eliminates the many custom design modifications that are made in the prior art as a matter of routine.
The surfboard is typically constructed without a mold. The board is individually hand-shaped from a polyurethane foam blank, and the fiberglass and resin are applied by hand over the shaped foam core. The process is labor-intensive, requires considerable skill, and involves structural problems that dictate dividing the production process into two separate steps, with the foam blank supplied by a separate manufacturer.
To enhance the strength of the foam, the blank is molded in an extremely strong, heavy mold made of reinforced concrete. This allows an excess of liquid pre-foam to be poured in the mold; as the foam expands, the excess compresses under high pressure against the surface of the mold and produces a density-gradient in the blankxe2x80x94the foam is soft and weak in the center and becomes progressively harder and denser towards the surface. To avoid removing too much of the harder, denser surface foam during shaping, the blank is molded close-to-shape, or as thin as possible. This close-to-shape molding has the drawback of increasing the requisite number of molds for surfboard production, and frequently leaves insufficient foam in the nose and tail areas of blank for the shaper to produce the desired lengthwise bottom curvature or rocker in the board.
The molded in rocker of the blank must therefore be modified by the blank manufacturer by gluing the blank to a wooden center spar or stringer cut to dimensions specified by the customer, and usually selected from a list of stock lengthwise rocker modifications. Clark Foam of Laguna Niguel, Calif., (www.clarkfoam.com) lists in its Rocker Catalog the dimensions of over two thousand different templates available to modify the lengthwise curvature of the more than sixty blank molds offered for surfboard production. Producing density gradient in the foam and producing the frequent lengthwise rocker modifications are necessary to maintain an adequate level of strength on the board significantly increase the costs of production but are essential, because the board""s impact resistance is very low.
The single fiberglass ply used on the bottom of the board will usually dent or fracture with moderate finger/thumbnail pressure, while the double or triple layer used to reinforce the deck (or top surface of the board) in the tail area where the rider stands often fatigues, becomes permeable to water, then fails and completely delaminates under the repeated high pressure of the rider turning the board. Shaping also limits the effectiveness of the longitudinal reinforcementxe2x80x94it makes wood the material of choice for the center spar and also makes it impractical to add top and bottom spar capsxe2x80x94the lack of effective longitudinal reinforcement leaves thinner in particular susceptible to breakage.
The fundamental problem is the one-to-one weight ratio of skin material to interior core. With current methods of production, the only practical method of improving this ratio and enhancing the overall strength-to-weight ratio of the board is to use the second and more expensive of the two basic methods of molded sailboard construction outlined briefly below.
The methods of molding that offer very low overall costs of production typically employ a blow-molded or thermoformed skin of pure plastic, or lightly foamed, fiber-reinforced plastic, and generally produce boards with excessive weight or an inadequate level of strength. An illustration of low production costs, however, is provided by the U.S. Pat No. 4,713,032 to Frank, the specification of which is incorporated herein, which uses quick-setting foamed polyurethane resin in the skin to achieve a rapid mold-cycle of about twenty minutes per board and high production from the molding tool of as many as twenty-four boards per day. The light foaming of the resin matrix greatly reduces the tensile strength of the reinforcing fiber, however, which leaves strength-to-weight and skin-to-interior core ratios well below expensive high performance sailboards that employ a xe2x80x9ccored compositexe2x80x9d or xe2x80x9cstructural sandwichxe2x80x9d skin.
The core in the structural sandwichxe2x80x94usually a thin layer of high-density plastic foamxe2x80x94spaces apart the two layers of high-strength laminate on either side so that the skin delivers the strength and stiffness of much thicker material, but at a fraction of the weight. The sandwich skin is expensive to fabricate because of the very long mold cyclexe2x80x94vacuum pressure is used to cause the skin material to conform to the shape of the mold, to prevent any spring-back of the skin core the material must remain in the mold under vacuum pressure for about two to three hours, until the resin has completely cured. The added drawback is that the difficulty removing excess resin from the laminate usually prevents the skin structure from attaining even higher strength and a further reduction in weight.
For example, when the laminate is saturated in the shallow, concave interior of the opened mold, the mold""s shape is a problem because the sharp edge contours create a dam, and the addition of the sheet foam skin core layer creates a buffer that significantly reduces the effectiveness of the squeegee on the interior layers of laminate. When the skin is fabricated first, the core of the board is formed by injecting liquid polyurethane pre-foam into the interior of the closed mold, the drawback being that this requires an extremely strong mold; the halves are typically attached to steel reinforcing jigs and held in a hydraulic press to prevent the mold from distorting, buckling or separating under the high pressure.
A more common method of sandwich skin fabrication is to use a lighter weight, pre-molded core of EPS (expanded polystyrene bead) foam. In this method the wet epoxy laminate/PVC sheet foam of the sandwich skin fits into molded-in recesses in the EPS core and the entire assembly is placed in the mold, the exterior of which then precludes resin removal by hand. Vacuum is applied to press the components tightly together and squeeze excess resin out in the process, but the pressure is limited to about 12-15 inches of Hg to prevent the mold from distorting and the foam core collapsing. Full vacuum (27 in. Hg) can be applied using the mold disclosed in the U.S. Pat. No. 5,023,042 to Efferding, the specification of which is incorporated herein, which provides an evenly flexing upper mold half that eliminates distortion problems by creating a completely even, permanent compression set of about three sixteenths of an inch in the finished board. This requires an interior core of uniform density, however; internal shear webs, compression inserts in the tail area, or hollow, weight-reducing chambers in the interior are problematic, as they tend to exacerbate distortion problems under vacuum. The additional problem in the above techniques is that the skin core generally will not conform to the sharp, compound curvature at the rail; the loss in strength caused by the gap in the sandwich structure at the perimeter edge is made up by adding many extra layers of laminate or molding-in an inward turning rail flange, both of which add extra labor and unnecessary weight.
U.S. Pat. No. 4,065,337 to Alter et al., the specification of which is incorporated herein, describes a process for molding structural members from plastic sheets for use in forming boat hulls.
At the time of the present invention, the board making arts had need of a method of high-strength sandwich skin fabrication with a mold-cycle sufficiently reduced to make production costs competitive with the low-cost pure plastic, or lightly foamed, fiber-reinforced plastic skin methods of production outlined above. In this invention, the method of producing such a skin has led to the development of a mold that has the capacity to produce custom or identically shaped boards. Accordingly, the present invention has the potential to eliminate the serious structural shortcomings and productivity constraints found in both the hand techniques and molded methods of the prior art.
In an embodiment of this invention, the board comprises right and left halves; the structural exterior skin in each half is a thin monocoque shell composed of a layer of high-density plastic foam and layer(s) of fiber reinforced plastic composite laminate. The board""s exterior skin encompasses low-density foam, chambered foam, honeycomb foam or a generally hollow interior; the two halves are bonded together at or about the longitudinal axis of symmetry, to a high-strength composite spar in a box- or I-beam configuration.
A mold used to fabricate the above board is divided into right and left halves, and each mold half then comprises two additional components: a substantially flexible female surface imparts a smooth surface to the exterior skin, and a rigid mold component defines the shape of the board by means of vacuum-forming a thin layer of skin core and high-strength facing material to form a complete exterior skin.
When molded, the thin layer of material in the skin core has sufficient thickness to bridge small gaps and mask minor imperfections on the surface of the mold, allowing the shape-defining mold to be divided into separate parts designed to be moved, then fixed and set to change dimensions, describe different curves or modify other parameters of the board""s design. In the present invention, for example, the bottom panel of the mold is designed to bend lengthwise to alter the rocker curvature of the board, the deck panel is adjustable to control thickness, a flexible, articulated rail component fits between the two to modify the outline and width, and nose and tail components, designed to slide fore and aft for changes in length, complete either end. Reference for the movement and fixed attachment of the mold parts is provided by a mold base, placed parallel to the mold""s longitudinal axis of symmetry, and/or each other.
When fixed attachment is to a mold base, the exterior or male surface of the mold is used, and the mold thus configured and can accommodate virtually all the common modifications required within a particular style of board. The added compound curvature of certain design features, however, can limit the bending capacity of the affected mold panel or rail component sufficiently to make further smooth bending difficult if not impossible. These cases are accommodated by an alternate embodiment of this invention, in which the mold components are reversed and attached to an external frame, thereby creating the concave cavity of a female mold. After the rocker, thickness and various design parameters of the mold are set, foam is molded in the cavity to produce a foam blank that, upon removal, can quickly have the desired features shaped by hand; the shaped blank then provides the (male) mold needed to form the thin layer of core in the structural exterior skin. The male/female configurations of the mold permit the fabrication of a wide array of custom designs, and the board thus produced has a high-strength structural sandwich skin with low costs of production.
In a method of forming the board, skin is formed by placing a sheet of high-density thermoplastic foam between a mold and an airtight bag; after heating the foam to its deformation temperature air is withdrawn from the bag, whereupon vacuum pressure causes the layer of foam to conform to the shape of the mold, creating a thin foam shell that then defines the exterior shape of the board in the subsequent steps of production.
When laminate is applied to the skin core, its pre-molded shape gives sufficient form and stability that it performs the normal function of the moldxe2x80x94it provides an accurate shape in the laminate as it hardens and cures. Because the original shape-defining mold no longer plays a critically important function in this role, the laminating step is designed so that the thin, flexible female component, or another thin layer of material, eliminates the bond between the wet laminate and the surface of the mold, while a third member is attached or temporarily combined with the skin core so the laminate/skin core assembly can be removed from the mold and set aside to cure, thus reducing the mold-cycle to the time needed to initially pre-mold the skin core and apply the laminate, under ten minutes per side in each operation. The technique is adaptable to the conventional female mold of the prior art, though the broad, flat pre-molded skin cores in top and bottom halves will readily bend of their own weight, making the additional support provided by an EPS core or longitudinal spar desirable throughout the cure. The skin core is pre-molded into a thin shell that forms an arch lengthwise and in cross-sectionxe2x80x94the stability provided by its arcuate shape provides significant manufacturing benefits and produces an exceptionally strong, lightweight board as well.
The board is laminated with the width at right angles to the worktable, where gravity provides a very effective aid in removing excess resin. The thin, flexible female surface allows substantial squeegee pressure to pass through to the laminate, gravity then removes the restxe2x80x94unobstructed, the resin quickly runs off the smooth, vertical sides of the molded foam shell, leaving an absolute minimum within the fiber. The higher strength of the laminatexe2x80x94combined with the greater shear strength of the seamless monocoque railxe2x80x94allows a further reduction in the weight of the interior core. In addition, the bond-line and its attendant reinforcement are moved away from the perimeter edge and placed along the longitudinal centerline of the board, and used to create a high strength composite spar that guards against board breakage. Other advantages include a more efficient use of spacexe2x80x94the board""s thickness rather than its width occupies manufacturing area during constructionxe2x80x94and the major labor savings possible using a mechanical fabric-impregnator to quickly pre-saturate the fiberglass cloth. These, and other structural advantages and manufacturing benefits will be more fully explained and better understood given the context provided by the detailed description of the invention, and upon viewing the drawings.